1. Field of the Invention
Embodiments disclosed herein relate generally to the production of polyolefin foams. Specifically, embodiments disclosed herein relate to polyolefin dispersions, froths, and foams containing flame retardants and/or phase change materials.
2. Background Art
The physical and mechanical properties of polymeric foams make them suitable for a wide variety of applications such as fire barriers, absorbent articles, sound deadening, thermal insulation, sports protective equipment, and packaging materials. There are six basic types of foams and foam materials: open cellular, closed cellular, flexible, rigid, reticular, and syntactic. Open cellular foams have interconnected pores or cells and are suitable for filtration applications. Closed cellular foams do not have interconnected pores or cells, but are useful for buoyancy or flotation applications. Flexible foams can bend, flex or absorb impacts without cracking or delaminating. Reticular foams have a very open structure with a matrix consisting of an interconnecting network of thin material strands. Rigid foams feature a matrix with very little or no flexibility. Syntactic foams consist of rigid microspheres or glass micro-balloons held together by a plastic or resin matrix. A burgeoning area within foam technology is the development of flame retardant foams to meet the demands imposed by stricter governmental standards for flame retardant articles.
The most common method of decreasing the flammability of polymeric foams is to incorporate a flame retarding agent, such as a halogenated compound or a phosphate ester into the foam formulation. While such compounds provide some improvement in the flame retardation properties of the foams, the incorporation of these materials may impair other foam properties. For example, in the upholstered furniture industry, where there has been an increase in the stringency of governmental flame retardancy standards, conventional flame retardant systems often degrade the soft feel of the fabric due to an increase in stiffness associated with incorporation of the flame retardant system.
Because combustion requires air, closed cell foams have been frequently used in flame retardant applications due to the limited amount of accessible combustible air entrapped in the closed cell foam material. Typically, closed cell foams are formed using gas blowing agents other than air, such as fluorohydrocarbons, to create the foam structure. However, because of the closed cell structure, the limited volume available in closed cell foams restricts the amount of flame retardant additives that may be incorporated while maintaining the integrity of the foam structure. Furthermore, when applying a flame retardant foam to fabrics, where a soft feel may be important to a consumer, closed cell foams are generally considered less desirable as they tend to be stiff.
Additionally, the closed cell structure may limit the type of flame retardant additives that may be incorporated, and may also limit the methods by which flame retardants may be incorporated into the foam. For example, closed cell gas blown foams may be made flame retardant via the incorporation of flame retardant additives such as brominated, chlorinated, or phosphorous based materials. The amount of these flame retardant materials that may be incorporated is limited in some cases by the compatibility of the material with the polymer being foamed. Gas blown foams require good film forming properties in order for a foam to be formed. Use of flame retardant additives that are particulate in nature or incompatible with the foaming material may interfere with the film forming properties, making it difficult to forty) a good quality foam.
Another disadvantage of using a closed cell foam in a flame resistant application is that closed cell foams often do not shrink away from the flame source. Because of the trapped gas in the closed cells, closed-cell foams may expand toward the flame providing a good fuel source for the fire.
Open celled foams may be formed by secondary processing of closed cell foams. This may provide for the use of additional methods for incorporation of flame retardants into the foam structure, with limitations known to those in the art.
In contrast to the stiffer closed cell foams discussed above, open cell foams possess the quality of elasticity and soft feel that consumers desire in fabric materials. Open cell structures are generally formed using water (steam) as the blowing agent with air comprising the majority of the void space of the final foam structure. While the open cell structure entraps significant amounts of combustible air, the larger voids provide greater surface area and volume to incorporate greater quantities of flame retardant fillers and other additives. Importantly, the open cell structure may accommodate larger amounts of these additives without compromising the foam structure.
There exist several methods for incorporating flame retardants into foams. For example, flame retardants have typically been incorporated into traditional blown foams by a dry blending process, such as that described in U.S. Patent Publication No. 20040138351. In the '351 publication, polyethylene was dry blended with a variety of possible melamine and organohalogen or organophosphorus flame retardant compositions, and the pelletized blend was then blown into a foam.
In U.S. Pat. No. 5,132,171 an open cell foam containing flame retardants is disclosed. The open cell foam is formed by subjecting a closed cell flame incorporating flame retardants to mechanical compression to rupture the cell membranes and result in an open cell structure. A second flame retardant may be also impregnated in the open cell structure by immersion of the foam in a solution containing the second retardant and wringing out the excess solution. This two step introduction of different retardant agents led to a synergistic improvement in flame retardation.
Another strategy for introducing flame retardants is disclosed in U.S. Patent Publication No. 20010006865 wherein a flame retardant gel-coating is placed over foamed polymeric material. The process can be used with either closed or open cell foams, however, the advantage of gel coating an open cell foam is that the entire foam structure becomes impregnated with the gel-coating through an immersion and wringing process.
A final challenge in the formation of foams is inconsistent and undesired foam collapse during the drying process, thus making the properties of the foam difficult to control. Further complicating this problem may be the presence of surfactants and flame retardant additives which can impact the final foam structure.
Accordingly, there exists a continuing need for the development of foam technologies to enhance flame retardant properties while preserving the basic loam function.